Before cutting the discussion as to when Union Cross' story will end: take into consideration that Dark Road is tied to KHUx in terms of accessing the game. If anything, Ux will end alongside DR, meaning that Union could very well end during the last months of next year, seeing as how I don't think Dark Road will have that much of story content.
Honestly, I think the only reason why Dark Road is tied to the Union Cross application is because it keeps the existing UX playerbase as oppose to forming a new one with a new app. Dark Road was created because the UX developers wanted to work on another Kingdom Hearts project after UX, meaning, Union Cross is finally being finished.